To find rational approaches for managing hazardous volatile organic contaminants (VOC's) in the environment, the processes that control their movement in groundwater systems must be understood. Little is known about VOC movement in the unsaturated zone. The overall goal of this project is to characterize the processes that control movement of VOC's in unsaturated natural soils. One set of experiments will be performed to determine if saturation- pressure relationships for any tow-phase fluid system can be predicted from the measured relationship for a particular two-phase fluid system using appropriate interfacial tension measurements. The adequacy of the method in real soil, in soils containing organic material, and in soils containing trace amounts of other contaminates will be investigated by measuring two-fluid pressure-saturation relationships in an air-water- NAPL fluid system. A second set of experiments will be performed to quantify the role of different soils to act as temporary or permanent sinks for dissolved VOC's passing through unsaturated natural soils. The measurements will relate retardation factors determined in the analysis to soil composition, looking especially at the importance of organic content, soil chemistry, soil water content, and the solution influx rates. These experiments will generate data suitable to calculate the dispersivity that describes a media characteristic during unsaturated flow. These dispersivities will be compared to dispersivities measured under saturated conditions to determine how sensitive the values are to the unsaturated state of the media. A third set of experiments will be conducted to provide data for validation testing of numerical model capability to predict the movement of VOC's through the unsaturated zone. Existing or improved numerical models will be tested. A fourth set of experiments will take advantage of the information already developed on flow in natural soil columns. Similar columns will be used to determine how water flux may be controlled 1) to induce microorganism movement through selected soil zones and 20 to cause subsequent attachment of the organisms in a previously selected soil zone.